Conventional power line frequency measurement circuits rely on analog circuits to determine the frequency of a measured signal. For example, an analog circuit may be used to detect a beat frequency between a reference signal having a known frequency and the frequency of the signal being measured. The beat frequency represents the frequency difference between the reference signal and the measured signal. By adjusting the frequency of the reference signal to eliminate the beat frequency, the frequency of the measured signal can be determined. Other analog circuit measurement methods use a phase-locked loop to determine the frequency of line signals. These circuits can be complex and require relatively expensive components to achieve accurate measurements. Such components also tend to fail over time. What is needed is a more reliable and robust, low cost and simple but accurate method of determining the frequency of line signals.